Electrical resistor



' Feb. 20, 1968 R. 's. MARTY ET AL ELECTRICAL RESISTOR Filed May 27,1963 INVENTORS I Robert .Mar

John H Fabmcuw %Wg ATTORNEYS United StatesPatent O 3,370,262 ELECTRICALRESISTOR Robert S. Marty, Nashua, N.H., and John H. Fabricius, Stamford,Vt., assignors to Sprague Electric Company, North Adams, Mass., acorporation of Massachusetts Filed May 27, 1963, Ser. No. 283,176Claims. (Cl. 338309) This invention relates to a new and improved metalfilm resistor, and more particularly to a preclous metal film resistorhaving a thin resistive film of metal formed 'on a surface in amonolithic sandwich configuration.

Thin metallic films used as resistances may be produced by firing frommetallo-organic formulations. The resistive into a structure which willpermit it to be used in normal electronic and electrical usages. Thethin resistive film must be supported on a base. It must be suitablyprotected against mechanical damage, and it must receive electrodeswhich complement the properties of the film. It is also advantageous toincorporate the thin film in a single composite structure which containsmore than one individual electrical component. For example, it isdesirable to mount the resistive film on a rigid base material carryinglayers including as an active component a resistive metal film.

Although they are advantageous, the resistor constructions made up oflayers on an inert base and carrying a metal resistive film suffer fromseveral distinct disadvantages. The layers do not adhere satisfactorilyto the inert base material. The plurality of coatings have a tendency toseparate from the base and from each other.

It is an object of this invention to provide a resistor constructionhaving a resistive film contained within layers on an inert base andhaving superior mechanical and electrical stability.

It is another object of this invention to produce a precious metal filmresistor with high resistivity and good electrical properties.

Still other objects of this invention will become apparent when read inconjunction with the following description and accompanying drawings inwhich:

FIGURE 1 is a plan view of a resistor construction of this inventiongreatly enlarged;

FIGURE 2 is a sectional view of a build-up of the resistor constructiontaken on lines II-II of FIGURE 1 in the direction of the arrows, and

FIGURE 3 is a sectional view of a higher resistance buildup of alternatelayers of vitreous glaze and resistive film greatly enlarged.

In general this invention involves a resistor structure made up of athin resistive film deposited from a metalloorganic composition on afused inorganic non-conducting layer. After the organic resinate isburned out of this composition the remaining metal resistive filmbecomes the electrically active portion of the resistor. Finally, overthis resistive film is lain an overcoat of a glaze similar to the underlayer and having substantially the same or a slightly lower coefiicient.The resistive film is provided with suitable electrodes. In a furthergeneral aspect, an inert base carries the fused inorganic non-conductinglayer which has a lower coeificient of expansion under thermal changethan the inert ceramic base.

Referring to the invention as illustrated in FIGURE 1,

3,370,262 Patented Feb. 20, 1968 a fiat ceramic substrate 10 such asbarium titanate in a suitable form has applied to a surface thereof anundercoat 11 composed of a vitreous glaze. The glaze of undercoat 11 ismade up of a suitable mixture of vitreous materials such asboro-silicates mixed with Pyrex glass to have a coefi'icient of linearexpansion with temperature lower than the ceramic of the substrate 10 inthe temperature range of from room temperature to 1100 C. Suitableelectrodes 12 composed of a non-diffusing material such as platinum-goldalloy are spaced apart in narrow strips on the undercoat 11. A thinresistive film 13 composed of an alloy of precious metals and basemetalextends across the undercoat 11 from one electrode 12 to the other. Anovercoat 15 of vitreous glaze covers the resistive film 13 and adjacentportions of electrodes 12. Suitable leads 14 are attached to theuncovered portions of electrodes 12. The vitreous glaze compositions ofthe undercoat 11 and the overcoat 15 may be of substantially the samecomposition with the composition of the overcoat 15 preferably having aslightly lower coefficient of expansion and a lower firing temperature.

FIGURE 2 shows the arrangement of the layers in section taken on lineII-II of FIGURE 1. FIGURE 2 illustrates the relationship between theelectrodes 12 which placed on the undercoat 11 are partially overlain bythe resistive film 13 and also the overcoat 15. The overcoat 15 extendslaterally across the resistive film 13 to contact the undercoat 11adjacent the resistive film 13 as well as elsewhere on the substrate 10.

Before applying undercoat 11, resistive film 13, and overcoat 15, thesurface of substrate 10 is suitably cleaned and prepared for thereception of the layers thereon. The undercoat 11 is preferably of afinely divided material such as a mixture of borosilicate with powderedPyrex glass. Suitable other inorganic non-conducting materials may beused. These may be applied by mixing with a suitable carrier or binder;the combination being applied either across the entire substrate surfaceor in a desired pattern by any desired process such as printing,screening, painting or rolling. For the purpose of the descriptionscreening is preferred. The preferred material of this embodiment is aborosilicate of a general formula Na O, BaO, B 0 A1 0 SiO mixed withequal parts of Pyrex glass. A suitable carrier may consist of 5% ethylcellulose and 95 pine oil. The overall mixture may be 25% carrier andfinely divided inorganic material depending upon the consistency desiredfor the method of the application being employed. All elements of thecarrier are evaporated or burned out during subsequent operations. Theundercoat 11 should provide a continuous layer for the resistive film 13and the adjacent portions of the electrodes 12.

The frit or mixture of carrier and inorganic material is applied to thesurface of substrate 10, as by screening. The preferred frit fuses attemperatures of approximately 1800-1900 F. As described at greaterlength below, the frit is selected to provide an undercoat 11 having alower temperature coeflicient of expansion than the substrate 10. Theapplied frit is fired on the substrate 10 at a temperature capable offorming a continuous film by fusion (1800-1900 F.).

One, two or three applications may be made to buildup the desiredcoating. An undercoat 11 for the resistive film 13 is thus produced onthe ceramic base 10.

In the preferred embodiment of this invention, the substrate 10 iscomposed of a barium titanate base material having a suitabletemperature ccefiicient of expansion. The following table sets forth thecoeificients for four typical barium titanate ceramic base materials,but is not intended to be limitative.

3 TABLE I Range of expansion Temperature range coefficient in 10 in C.:cm./cm./ C.

Ceramic:

(1) 27 to 1100 8.6 to 13.1 (2) 30 to 1100 6.6 to 12.7 (3) 25 to1100 9.1to 11.6 (4) 25 to 1100 6.9 to 13.2

A suitable vitreous glaze for the undercoat 11 is composed of one halfborosilicate and one half Pyrex glass and has an expansion coefi'icientof 5 x cm./cm./ C.; thus, an undercoat 11 having a lower coefficient ofexpansion is provided.

The electrodes 12 of platinum-gold alloy are applied by screen printingin a suitable manner. The electrodes are selected to minimize diffusioninto the resistive film. Suitable platinum-gold formulations which firein a temperature range of 1300l400 F. are next applied and fired in thattemperature range to form the electrodes 12. Any suitable patterncapable of providing termination of resistance film 13 may be employedfor electrodes 12. However, the dumbbell-like configuration shown inFIGURE 1 offers the advantage of economy of the costly electrodematerial.

Then a thin coating of an organic metallic compound is suitably appliedto the undercoat 11 across the electrodes 12 for the production of theresistive film 13. The organic metallic compound is made up of anorganic resinate of the alloy of the resistive film. Firing of the alloyprovides a resistive film having desired electrical characteristicsincluding preferably a zero temperature coefiicient of resistivity andis capable of providing a film in the order of 2 X 10* inches thick.

The organic resinate compound is applied to the undercoat 11 in anappropriate vehicle by dipping, brushing, spraying, or screening. Theresinate in the vehicle is applied as thinly as is necessary to give tothe deposited metal film the desired thickness and ohmic value ofresistance. The vehicle functions as a medium for carrying the resinateso that the resinate is evenly distributed upon the undercoat 11. Thatis to say, the organic metallic resinate is evenly dispersed in thevehicle, and when fired to the undercoat gives a good thin metal film.The resinates may include as constituents natural occurring resinates,resins and synthetic preparations. The resinates are precious metalcompounds with natural or synthetic resins or simple reactive organiccompounds. The metal resinate is suitably prepared for use in theproduction of the resistive film 13 by known methods as for example bysimple solution of resin in a base followed by addition of the preciousmetal salt.

After the coating comprising the metal resinate compound and its vehicleis applied to the glaze underlayer 11, the base 10 and underlayer 11,and the coating are given the first stage of heat treatment to decomposeby pyrolysis the binder and organic portion of the metal compound.Deposition of the metal from the resinate starts at temperatures ranginganywhere from approximately 200 to 400 C. Crystal formation of theprecipitated metal occurs at a temperature in this range and progresseswith time until approximately 100% metallic deposit results. The timewas found to vary anywhere from 15 to 30 minutes. Accompanying theprecipitation of the metal is the deposition of some carbon and basemetal oxide ash from the binder and flux or frit respectively.

The second stage heating is to completely oxidize the ash or residue andto ensure a thorough precipitation of the precious metal film. Thesecond stage heating may range from 400 to 750 C. for about one hour.FIGURES 1 and 2 show the deposit thoroughly oxidized leaving the thinmetal film 13 which may be characterized as the basic resistance. Whilemetal film 13 is shown in FIGURES 1 and 2 as having a straight-linepattern between electrodes 12, other patterns, such as zig-zag paths,are utilized when increased resistance is required. The metallic film 13possesses good bonding properties so that the underlayer 11 and thedeposited metal are virtually integral.

Other well-known techniques for using metal resinates are similarlysatisfactory. The metal resinate film is fired at about 1300 F. toproduce a resistive film. The organic portion of the resinate and thevehicle are driven 011.

The metal composition is made up of a relatively large proportion of analloy of precious metals and a relatively small proportion (up to 20% ofthe precious metals) of a base metal, such as bismuth oxide. Thisinvention is described in connection with a series of precious metalcombinations containing gold or iridium, platinum and rhodium, but it isunderstood that it includes their equivalents. Palladium may be includedin the previous metal alloy to increase the resistivity of the alloy.

To illustrate the resistive films the following proportions are givenfor desirable organo-metallic formulations, but as exemplificationsonly. The percentages in these examples are by weight.

The precious metal component of Formulation I is composed of 76.8% gold,19.2% platinum and 4% rhodium. A modification may be made in the goldcontent by replacement by an alloy of from 70-90% gold and 1030%palladium. About 5.1% of the total resinate composition is preciousmetal. Base metal oxide is present in a minor proportion that is about19.6% of the precious metal.

The precious metal component of Formulation II may be 69% platinum,29.8% iridium and 1.2% rhodium. The precious metal is about 2% of thetotal composition, and base metal oxide is present to the extent ofabout 10.5% of the precious metal.

The overcoat 15 of vitreous glaze is applied over the resistive film 13and adjacent portions of electrodes 12 to come into contact with theundercoat 11. The overcoat glaze is selected to fire at a temperature ofabout 1100 F. In the final step the overcoat 15 is fired at about 1100F. and the glazes of the undercoat 11 and overcoat 15 tend to react toproduce the monolithic sandwich structure of this invention.

Among other features of this invention the vitreous glaze of: theundercoat remains relatively hard at the temperature of 1300-1400 C.during the firing of the resistive material. Further, the undercoat andovercoat are under compression and it has been found that this providesimproved mechanical stability in the metal film resistor structure. Theresistance has a temperature coetficient of resistivity which is closeto zero.

FIGURE 3 illustrates a modification of this invention. In FIGURE 3, alayered structure is made up of glaze layers 15 and resistive films 13on an underlayer 11. The resistive structure is made up by firstapplying the underlayer 11 to the substrate 10. The underlayer 11 isthen fired on the base as described above. The layered substrate base isthen removed from the oven and an electrode 12 (as in FIGURE 2) isapplied adjacent the left side as seen in FIGURE 3. Electrode 12 isapplied adjacent the right side of layer 11 and does not extend ontosubstrate 10, inasmuch as 12' is not a terminal electrode. Theelectrodes are suitably fired as described above. In the next step, themetal resinate is applied overlying the electrodes 12 and 12' and isfired to form resistive film 13. Overglaze 15 is then applied to coverfilm 13 and adjacent portions of electrodes 12 and 12'. Anotherelectrode 12" is applied over a part of glaze 15 to provide anotherintermediate electrode permitting the continued build-up of theserpentine resistance path. The layers and films 15 and 15" and 13 and13" are then applied and fired successively in the manner describedabove. Suitable electrodes 12" and 12" are also laid down and fired andreceive the respective resistive films 13' and 13". The resistive films13 and 13" are sandwiched between the overcoats 15, 15' and 15" and eachresistive film extends from its respective electrode 12, 12 and 12" tothe opposite side in a continuous zig-zag path. Conventional leads 14are attached to the electrodes 12 and 12".

The invention has been described with particular reference to specificembodiments thereof including modification; however, it will beunderstood that it is susceptible to embodiment in a large number offorms still within the spirit of this invention. For example, thearrangement of the resistive films and their electrodes may berearranged and the application of the resistive film need not be throughevaporation of a solvent from the metal resinate, but may be produced byother techniques. Other ceramic substrates such as alumina may also beused. Accordingly, the scope of this invention is limited by theappended claims.

What is claimed is:

1. In an electrical resistor, an inert electrically nonconductive basehaving a coefficient of expansion of over 6 l cm./cm./ C. over atemperature range of from 27 C. to 1100 C., a fired-on continuousinorganic homogeneous electrical non-conducting first layer on a surfaceof said base having a lower temperature coeflicient of expansion thansaid base, a fired-on electrically resistive metal film on said firstlayer having a firing temperature not greater than said first layerwithspaced electrodes in electrical connection with said film, and afired-on continuous inorganic homogeneous electrically non-conductingsecond layer overlying said film and adjacent portions of saidelectrodes and joined to said first layer in areas of contiguity, saidsecond layer having a firing temperature not greater than said film anda temperature coefficient expansion not greater than first layer andsaid base.

2. An electrical resistor as claimed in claim 1 wherein said first layerand said second layer are comprised of borosilicate.

References Cited UNITED STATES PATENTS 2,357,473 9/1944 Jira 338-308 X2,521,894 9/1950 Brown 338-314 X 2,808,351 10/1957 Colbert et al.338-308 X 2,927,048 3/l960 Pritikin 117-215 3,134,689 5/1964 Pri'tikinet al. ll7-2l2 3,202,951 8/ 1965 Krinsky 338-2 3,217,281 11/1965 Greistet al. 338-309 3,244,559 4/1966 Sivertsen et al. 338-309 X 2,786,9253/1957 Kahan 338-262 2,859,321 11/1958 Garaway 338-262 X 2,939,8076/1960- Needham 117-212 2,950,995 8/1960 Place et al. 338-308 X2,950,996 8/1960 Place et a1 338-308 X 3,114,868 12/ 1963 Feldman117-217 X 3,248,680 4/1966 Ganci 338-266 FOREIGN PATENTS 8/ 1946 GreatBritain.

RICHARD M. WOOD, Primary Examiner.

VOLODYMYR Y. MAYEWSKY, Examiner.

1. IN AN ELECTRICAL RESISTOR, AN INERT ELECTRICALLY NONCONDUCTIVE BASEHAVING A COEFFICIENT OF EXPANSION OF OVER 6X10-6CM./CM./*C. OVER ATEMPERATURE RANGE OF FROM 27*C. TO 1100*C., A FIRED-ON CONTINUOUSINORGANIC HOMOGENEOUS ELECTRICAL NON-CONDUCTING FIRST LAYER ON A SURFACEOF SAID BASE HAVING A LOWER TEMPERATURE COEFFICIENT OF EXPANSION THANSAID BASE, A FIRED-ON ELECTRICALLY RESISTIVE METAL FILM ON SAID FIRSTLAYER HAVING A FIRING TEMPERATURE NOT GREATER THAN SAID FIRST LAYER WITHSPACED ELECTRODES IN ELECTRICAL CONNECTION WITH SAID FILM, AND AFIRED-ON CONTINUOUS INORGANIC HOMOGENEOUS ELECTRICALLY NON-CONDUCTINGSECOND LAYER OVERLYING SAID FILM AND ADJACENT PORTIONS OF SAIDELECTRODES AND JOINED TO SAID FIRST LAYER IN AREAS OF CONTIGUITY, SAIDSECOND LAYER HAVING A FIRING TEMPERATURE NOT GREATER THAN SAID FILM ANDA TEMPERATURE COEFFICIENT EXPANSION NOT GREATER THAN FIRST LAYER ANDSAID BASE.